An ink-jet recording apparatus that records an image by using an ink-jet recording head (hereinafter referred to as a recording head) discharging a minuscule ink droplet out of a nozzle causes an ink droplet to be discharged out of the nozzle by applying pressure to the ink in a pressure chamber and makes the ink droplet land on a recording medium such as recording paper.
Such an ink-jet recording method makes it possible to perform high-precision image recording with a relatively simple configuration and has rapidly evolved in a wide range of fields from a field for domestic use to a field for industrial use. In particular, various improvements in the enhancement of the speed and image quality of ink-jet recording have been proposed. While there is a strong demand for high-speed printing by the recording head, such as one-pass printing using a line head, there is also a demand for higher image quality by the enhancement of the reproducibility of gradations of a print image.
In the past, to enhance the reproducibility of gradations while performing high-speed printing, a method by which a plurality of ink droplets are discharged out of one nozzle per pixel has been adopted. However, when a plurality of ink droplets are allowed to be continuously discharged out of one nozzle, a longer printing time is required. When the pause period between the drive waveforms for each discharge of droplets is shortened to shorten the printing time, the discharge of ink droplets becomes unstable.
On the other hand, a method of enhancing the reproducibility of gradations of a print image by discharging a large droplet and a small droplet in one pixel has also been known. (for example, refer to JP-A-2002-86766 and JP-A-2002-321360)
In the method of discharging a large droplet and a small droplet, since different droplet sizes of the large droplet and the small droplet result in different sensitivity of the discharged droplet speed to a drive voltage, when the same power source is used, a difference in droplet speed is caused, resulting in displacements of the position in which an ink droplet lands. To address this problem, technologies disclosed in JP-A-2002-86766 delays the discharge timing as the droplet size (the amount of droplet) becomes large (as the droplet speed becomes faster) to prevent displacements of the position in which an ink droplet lands, the displacements caused by a difference in droplet speed between the ink droplets of different sizes, that is, the large ink droplet and the small ink droplet.
Moreover, technologies disclosed in JP-A-2002-321360 outputs a drive waveform by which an ink droplet with a medium volume is discharged before outputting a drive waveform by which an ink droplet with the largest volume is discharged and outputs a drive waveform by which an ink droplet with the smallest volume is discharged before outputting the above drive waveforms to prevent displacements of the position in which an ink droplet lands even when ink droplets of different droplet sizes are discharged.
The techniques of JP-A-2002-86766 and JP-A-2002-321360 are based on the premise that ink droplets of different droplet sizes differ in droplet speed and propose a method of adjusting the discharge timing for each droplet size to prevent displacements of the position in which an ink droplet lands. However, with this method, the drive period becomes undesirably longer due to a delay in the discharge timing, which presents a great problem in performing high-speed printing.
On the other hand, a method of making the droplet speeds of ink droplets of different droplet sizes uniform by varying the drive voltage at which the ink droplets of different droplet sizes are discharged is also adopted. However, this method makes a drive signal generating circuit complicated and increases costs.
Through an intensive study of different droplet speeds due to a difference in droplet size, the inventor of the present invention has found out that, by adjusting the pulse width of the drive waveform for a large droplet and the pulse width of the drive waveform for a small droplet, it is possible to make the droplet speeds nearly equal while allowing the large and small droplets to have different droplet sizes by using the same power source, and has made the present invention.